Regular electronic appliances normally use DC power as the source of their operating power, selectively use a battery to adapt to different operating environment or condition, or use the power rectified and filtered by a mains transformer. However, as the pattern using transformer occupies more space due to its huge size involved and the use of battery always incurs the inconvenience of frequent replacement or charging circuit implementation, how to directly and effectively acquire sufficient and stable DC power by chopping the AC mains electricity becomes the topics that power supply industry endeavors to develop and break through.
As for the chopping method of the current 50 Hz/60 Hz AC mains power, there are a multitude of designs and patent cases which have been disclosed already. For example, what is shown in FIG. 1 is a more advanced conventional serial chopper structure that is mainly connected with the live wire of a power supply in series to chop the power in an integrated manner and obtain a stable low-voltage DC power after rectification and voltage stabilization. The chopper structure includes: a rectification and voltage stabilization unit 91 outputting a stabilized voltage after rectifying the integrated chopped power obtained from the live wire of the power supply to provide a low-voltage DC power to internal circuit and external circuit; a stabilized voltage control unit 92 automatically outputting an integrated pulse wave when the integrated chopped power reaches a preset value; and an integrated chopping power driving unit 93 whose input end is connected with the stabilized voltage control unit 92 to receive the integrated pulse wave, thereby controlling the integrated chopping and generating an open-circuit chopped voltage with respect to the integrated pulse wave from the live wire of the power supply.
As shown in FIG. 2 which illustrates an output waveform when operating a conventional serial AC chopper, the chopping operation in the negative half cycle is still continuous. The ‘a’ portion on the bottom is consumed for reactive power. Temperature rise will arise while operating in a long duration. Meanwhile, the noise and interference to power will also increase. Whereas, after a capacitor stabilizes the voltage and outputs a waveform, the ‘b’ portion on the top will indirectly affect the voltage variation due to a load serially connected with an output power supply. As a result of the unstable DC power supply condition and the numerous limitations of peripheral products, the electromagnetic interference (EMI) out of reactive power becomes inevitable once an inductive load is connected.
Besides, most of the conventional serial AC chopping methods currently lack of the synchronous design feature, and the chopping circuit and the operation thereof still present the following drawbacks which are further to be improved anxiously:
1. In view of the chopping method without the synchronous design feature, the DC portion of the chopping output will always impact on the operational control of the subsequent connected MCU (Microcontroller Unit) to result in an abnormal operation.
2. As most of the chopping designs have no synchronous chopping development, the conversion efficiency fails to be upgraded so that a lengthy operation of the chopping components will give rise to the phenomenon of temperature rise.
3. As the operation pattern having a serial-connected load is adopted, given no good synchronous control, the 50 Hz/60 Hz serial AC chopping is usually subject to surrounding static interference and causes a resulting malfunction. This simply adds more determination conditions to the MCU, which lead to lots of inconvenience arising from heavier operational load and slower response time in operation.
4. Owing to no synchronous function, in the course of the AC chopping, phase will vary with the serial-connected load, such that a current reference point cannot be precisely set up. Therefore, when operating under a heavy load, the over-load protection fails to be brought into play and this issue becomes even more serious when connected with an inductive load.